In medical and industrial flexible fiberscopes, an image is transmitted using a bundle of coherently aligned optical fibers, which form a fiber bundle. The number of fibers in a fiber bundle varies from 1000 to 50,000 depending on the application and the size of the bundle. Each fiber, which can represent a pixel, transmits a small portion of the image, and the combination of the images from each fiber generates the whole image. To do so, coherent alignment of fibers in the fiber bundle is required in the distal and proximal ends of the bundle in order to correctly transmit the image from one end to other end. The distal end corresponds to the end where the image is located and the proximal end corresponds to the end where the image is observed.
FIG. 1 is a depiction of a fiber bundle 100 with a proximal end 101, a distal end 102, and a transition region 103 spanning the proximal and distal ends. Fiber bundle 100 comprises a plurality of fibers 104-110 that are coherently aligned. Coherent alignment means that the position of a fiber relative to all other fibers at one end should be the same as the other end. As such, the measured distances separating the center points of each fiber relative to all other fibers will be the same at the proximal and distal ends of the fiber bundle. For optical fibers, while it is possible to have coherently aligned fibers spanning the entire length of the fiber bundle, it is acceptable to have non-coherently aligned fibers in transition region 103.
For fiberscopes, the image fiber bundle needs to be flexible (i.e. bendable) between distal and proximal ends so that a flexible fiberscope can be bent to enable observation through curved openings or channels. It is well known that fiber can be bent to certain bend radiuses before breakage. When a glass fiber is bent, a tensile stretching force is created in the fiber that is inversely proportional with the bending radius of the fiber. If the tensile force is larger that the tensile strength of the fiber, the fiber will break. A broken fiber means a dark spot in a transmitted image. Typically, a glass fiber will be broken when it is bent to a bending radius of 100-500 times its own radius. Therefore, the smaller the fiber diameter the smaller bending radius that can be achieved.
In order to achieve the objectives of coherently aligned fibers and fiber diameters that are small enough to be able to bend the fiberscope without breaking the fiber, etchable fiber bundles have been utilized. FIGS. 2A-2E depicts the method by which etchable fiber bundles 150 are formed. At step 1(a), individual fibers 151 are brought together to form a bundle with a proximal end 152, a distal end 153 and a transition region 154. The fibers 151 are at least coherently aligned at the proximal 152 and distal 153 ends. At step 1(b), the fibers 151 are coated with an acid soluble glass 155 and baked so that fiber bundle 150 is rigid and the individual fibers 151 are thermally bonded together. At step 1(c), the proximal 152 and distal 153 ends of bundle 150 are covered with an acid resistant coating 156 that will protect these thermally bonded ends from acid. At step 1(d) the bundle is put into an acid bath to dissolve the glass 155 in the transition region 154. The proximal 152 and distal 153 ends of bundle 150 are protected by the acid resistant coating 156. At step 1(e), the acid resistant coating 156 is stripped away with a solvent. The result of this process is a fiber bundle 150 with rigid ends 152, 153 of coherently aligned fibers 151 and a flexible portion over the entire transition region 154 between the ends with independent individual fibers. The ridged ends provide the coherency while separated fibers provide the flexibility (ie. bendability).
Fiberscopes are known to be manufactured to different lengths and utilize fibers of different diameters. Typically, individual fibers have a diameter from 5 to 15 micrometers. The fiber bundles, which could have 1,000 to 50,000 fibers, are typically 0.2 mm to 2 mm, depending on the application.
One problem with image fiber bundle manufacturing techniques is the relatively high production cost and relatively low process yield. Image fibers are brittle and susceptible to breaking during the manufacturing process. An acceptable fiberscope should not have more than two broken fibers in the central area of a fiber bundle because a broken fiber will not transmit an image, but will be seen as a dark spot in the view. Given the number of fibers in a bundle (1,000 to 50,000), the length of the bundle (from 100 mm to 3000 mm) and small size of a fiber (from 5 micrometers to 15 micrometers), it is very challenging to manufacture a fiber scope with no broken fibers. For example, a 1.0 mm fiber bundle with 200 mm length and 20,000 fibers will be scrapped at the end of a manufacturing process if it is found that it has more than 2 broken fibers (i.e. less than 0.01% of the fibers are broken). Giving the probability of high scrap rates, keeping the manufacturing cost of fiberscopes down and delivering fiberscopes in a timely manner can be challenging.
What is needed is a fiberscope that can be manufactured at a reduced cost. It would be beneficial if such a fiberscope can be manufactured utilizing a process that minimizes the scrap rate or increases the process yield of a fiber bundle. It would also be beneficial if such a process also minimized the cost associated with scraping a fiber bundle.